This invention relates generally to optical communication systems. More particularly, this invention relates to wavelength division multiplexing (WDM) systems.
Optical wavelength division multiplexing (WDM) has gradually become the standard backbone network for fiber optic communication systems. WDM systems employ signals consisting of a number of different wavelength optical signals, known as carrier signals or channels, to transmit information on optical fibers. Each carrier signal is modulated by one or more information signals. As a result, a significant number of information signals may be transmitted over a single optical fiber using WDM technology. As the volume of traffic increases, more and more channels must be utilized. However, the bandwidth of the electromagnetic spectrum suitable for optical communication is limited. Consequently, the spacing of adjacent has channels decreased. The decrease in channel spacing has led to the development of dense wavelength division multiplexing (DWDM) technology and ultra dense WDM (UDWDM).
Optical communications systems use components referred to generically as optical interleavers to combine, split, or route optical signals of different channels. Interleavers are described, for example, in U.S. Pat. No. 5,694,233, issued to Wu et al. on Dec. 2, 1997. Interleavers typically fall into one of three categories, multiplexers, de-multiplexers and routers. A multiplexer takes optical signals of different channels from two or more different input ports and combines them so that they may be optically coupled to an output port for transmission over a single optical fiber. A de-multiplexer divides a signal containing two or more different channels according to their wavelength ranges and directs each channel to a different dedicated fiber. An optical interleaver can spatially separate dense WDM (DWDM) or ulra-dense WDM (UDWDM) signals into two complementary subsets, each having twice the original channel spacing. A router works much the same way as a de-multiplexer. However a router can selectively direct channels according to control signals to a desired coupling between an input channel and an output port.
Interleavers and routers separate or combine optical signals in two distinct wavelength ranges. These two wavelength ranges may define two or more distinct optical channels. FIG. 1 depicts a typical spectral response for an interleaver that uses birefringent waveplates. In FIG. 1 transmission 100 is plotted as a function of frequency 102. The spectral response is characterized by one or more passbands 110 separated by a well-defined channel spacing. Alternating passbands may be grouped so as to transmit one set of channels 111, e.g. odd channels, and one or more complementary sets of channels, e.g. even channels (not shown). The even channels typically have passbands that lie in between adjacent odd channels and vice versa.
In a de-multiplexing mode, an interleaver module would direct the odd channels to one I/O port and the even channels to another. Known interleaver designs are capable of separating odd channels from even channels where adjacent even/odd channel pairs are separated by less than 50 GHz. By cascading two or more interleaver modules it is possible to couple each individual channel to a particular I/O port. This works well enough if the number of channels is relatively small. However, as the number of channels increases, the number of cascaded interleaver stages also increases. The optical losses associated with each stage add up and the total loss becomes prohibitive. A large number of cascaded interleaver modules also adds complexity and cost. Furthermore, a large number of cascaded interleaver modules can make the size of the interleaver apparatus prohibitive where space is at a premium.
To overcome this WDM systems have been developed to direct each individual channel to a particular destination. Such systems typically use technologies based on thin film filters (TTF), arrayed wave guides (AWG) or fiber Bragg gratings (FBG) to separate the channels according to the carrier signal frequency. However, these methods are typically limited to channel spacings on the order of 100 GHz or greater. It is expensive to manufacture WDM modules capable of handling smaller channel spacings. Unfortunately, as the amount of communications traffic increases, more channels are required and channel spacing decreases.
There is a need, therefore, for an improved optical communications apparatus and method that overcomes the above difficulties.
Accordingly, it is a primary object of the present invention to provide an apparatus for handling DWDM using WDM modules. It is an additional object of the invention to provide a scalable DWDM system capable of handling an arbitrarily large number of channels.
These objects and advantages are attained by a wavelength division multiplexing (WDM) apparatus and a WDM method. According to a first embodiment, the apparatus comprises an optical interleaver coupled to one or more wavelength division multiplexing (WDM) modules. In a second embodiment, two or more interleaver modules may be cascaded together to expand the channel handling capacity. In a third embodiment, two or more WDM modules are cascaded together to divide the even and odd channels into subsets. According to the method, the interleaver interleaves a first signal, containing one or more odd channels and one or more even channels with a second signal, containing a portion of the odd channels and a third signal containing a portion of the even channels. A characteristic frequency spacing between adjacent even channels in the second signal and/or a characteristic spacing between adjacent odd channels in the third signal is about two or more times the value of a characteristic frequency spacing of adjacent channels in the first signal.
The interleaver may optionally comprise one or more modules of any conventional type, such as modules based on birefringent walk-off elements. The apparatus may incorporate apodizing or wavelength-slicing elements to accommodate narrow, e.g. less than 100 GHz, channel spacing.